Sign In  |  Register Now  Why Register?

The use of the protection technique known as Intrinsically Safe Systems is becoming more and more common-place in hazardous locations in recent Code cycles. There are two main reasons for its popularity and use. First, the installation of an intrinsically safe system, as compared to explosionproof wiring and apparatus, is far less costly in most cases. Second, improved safety. Common uses for intrinsically safe systems and circuits include functions such as leak detection systems and product level indicators. Many people in the electrical industry are exposed to these systems — from the installer, to the inspector, to the end user. Each of these people need to know the operation of an intrinsically safe wiring system, essentially how it works and what is its purpose. So just what is an intrinsically safe wiring system and how does it work? This article will take a look at these systems and clarify those gray areas.

What is an Intrinsically Safe Wiring System?
To fully understand this type of wiring and these systems, one must first be familiar with the definitions and terms associated with them. A closer look at the National Electrical Code is in order here. First, an intrinsically safe system is defined in the Code as, "An assembly of interconnected intrinsically safe apparatus, associated apparatus, and interconnecting cables in that those parts of the system that may be used in hazardous (classified) locations are intrinsically safe circuits." It should be noted that "an intrinsically safe system may include more than one intrinsically safe circuit." See Section 504-2, the definition of an intrinsically safe system.

What are the electrical characteristics of an intrinsically safe circuit? Why is it a safer protection technique? An intrinsically safe circuit is a circuit in which any spark or thermal effect is incapable of causing ignition of a flammable or combustible material in the air under prescribed test conditions. One method of producing an intrinsically safe circuit is to route it through a zener diode barrier. (See Figures 1, 2, 3, and 4.) Intrinsically safe circuits are permitted in any hazardous (classified) location for which it has been approved. Intrinsically safe wiring is also the only wiring method allowed in a Class I Zone 0 location. See Section 505-15(a)(1).

Diagram (Fused) Zener Diode Barrier

Article 504 of the National Electrical Code covers intrinsically safe systems. It first appeared in the NEC in the 1990 edition, and has not changed much since its arrival. This article includes specific definitions to clarify the terms and requirements for these types of wiring systems. It should be noted that all other articles of the NEC apply to the installation of intrinsically safe systems, except as modified by Article 504. See Section 504-3.

Section 500-4 includes nine specific protection techniques that are acceptable for electrical and electronic equipment in hazardous locations. Section 500-4(e) lists intrinsically safe systems as one of those techniques, and states that intrinsically safe equipment and wiring is permitted in any hazardous (classified) location for which it is approved. The requirements of Articles 501 through 503 and 510 through 516 shall not be considered applicable to such installations, except as required by Article 504. It goes on to state that intrinsically safe apparatus and wiring shall be in accordance with the requirements of Article 504. For more information on intrinsically safe apparatus and associated apparatus for use in Class I, II, and III, Division 1, hazardous locations refer to ANSI/UL 913-1997.

What are the Minimum Code Requirements?
So let’s look at the minimum code requirements one would expect to follow to achieve a code-compliant, intrinsically safe wiring/system installation. One of the first considerations when dealing with any hazardous (classified) location is to determine the classified location. The NEC identifies some common hazardous (classified) locations, but sometimes the appropriate NFPA Standard must be referred to for establishing the boundary or extent of the hazardous (classified) location. Locations are classified based on the properties of the particular flammable vapors, liquids, gases, or combustible dusts, fibers, or flyings that may be present and the likelihood that a flammable or combustible concentration or quantity is present.

Documentation Requirements
A new requirement in the 1999 NEC in Section 500-3(b) is that all areas designated as hazardous (classified) locations shall be properly documented. This documentation shall be made available to those authorized to design, install, inspect, maintain, and operate electrical equipment in those locations. It is important that the authority having jurisdiction be familiar with recorded industrial experience, as well as with the standards of National Fire Protection Association (NFPA), American Petroleum Institute (API) and the Instrument Society of America (ISA). These may be of use in the classification of various locations, the determination of adequate ventilation, and the protection against static electricity and lightning hazards.

One of the first and most important points here is to direct the reader’s attention to NEC Section 500-5, Special Precaution, which states that "Articles 500 through 504 require equipment construction and installation that will ensure safe performance under conditions of proper use and maintenance." The fine print note cautions inspection authorities to exercise more than ordinary care with regard to installation and maintenance.

Listing Requirements
Once the hazardous (classified) location is established, the installation and inspection process can proceed. Section 504-4 requires that all intrinsically safe apparatus and associated apparatus be approved. These systems are typically listed by a nationally recognized independent third party testing laboratory as suitable for use.

Control Drawing Requirements
As with any listed electrical equipment it is important to follow the installation instructions provided by the manufacturer to ensure code compliance and an installation that is essentially safe. Section 504-10 includes wording regarding a control drawing and states that the intrinsically safe apparatus, associated apparatus, and other equipment shall be installed in accordance with that control drawing(s). In Section 504-2 a control drawing is defined as "A drawing or other document provided by the manufacturer of the intrinsically safe or associated apparatus that details the allowed interconnections between the intrinsically safe and associated apparatus." This simply means that the control drawing is essential for the installer to make a proper installation and that the inspector will also need this documentation to make a proper inspection. Intrinsically safe apparatus shall be permitted to be installed in any hazardous (classified) location for which it is approved and general purpose enclosures shall be permitted for intrinsically safe apparatus. The control drawing(s) can include requirements such as specific types of conductors or cables to be used with the system, types of raceways to install (metal conduit or rigid nonmetallic conduit, for example), and specific grounding requirements such as requirement for a grounding electrode connection in addition to the equipment grounding conductor. It is important that the control drawing(s) be utilized when installation, or inspections, or servicing operations associated with intrinsically safe systems and wiring are taking place.

Because of the electrical characteristic of the intrinsically safe apparatus and wiring, it shall be permitted to be installed using any of the wiring methods that are suitable for unclassified locations or as specified by the control drawings. It is important that the sealing requirements of Section 504-70 be followed, although explosion-proof seals would not be required here. See Section 504-70.

Separation Requirements
Another extremely important issue regarding installation and inspections of intrinsically safe circuits and systems is the separation requirements set forth in Section 504-30. The first requirement deals with conductors and cables and separation distance between those intrinsically safe conductors and cables. Spacing of at least 1.97 in. (50mm) shall be maintained and the cables shall be secured to maintain that spacing. This spacing shall be maintained between these intrinsically safe circuits and any nonintrinsically safe circuits.

Conductors of intrinsically safe circuits shall not be placed in any raceway, cable tray, or cable with conductors of any nonintrinsically safe circuit. Alternatives by exceptions in the Code will allow a space separation or grounded metal or approved insulating partition to provide the separation required by this section. See Section 504-30(a)(2) Exception 1. The problem here is simply that the intrinsically safe circuit is incapable of a thermal effect or of producing a spark large enough to ignite a combustible vapor or atmosphere that may be present where this circuit is installed. If spacing is not maintained between normal AC circuits, for example, the effects of capacitance coupling and induced currents from adjacent non-intrinsically safe conductors or cables can be imposed on the intrinsically safe circuit conductors and possibly render the circuit not intrinsically safe at that point and capable of atmosphere ignition.

If multiple intrinsically safe circuits are installed, the Code requires a separation between the different intrinsically safe circuits by either a grounded metal shield, in an approved shielded cable, for example, or by each circuit conductor having an insulation thickness of at least 0.01 in. One should use special precaution here when evaluating whether or not the separation requirements of Section 504-30 have been met. Noncompliance with this requirement can result in an installation being unsafe and becoming a possible explosion hazard.

Grounding Requirements
The grounding requirements for intrinsically safe apparatus and equipment are simply that all cable shields, enclosures, intrinsically safe apparatus, raceways, and associated apparatus, if of metal, shall be grounded. Some intrinsically safe systems require a connection from the system to a grounding electrode. This connection is in addition to a provided equipment grounding conductor. If a connection to a grounding electrode is required, it shall be as specified in Section 250-50(a), (b), (c), and (d) and shall comply with Section 250-30(c). Section 250-52, Made Electrodes, shall not be used if any of the electrodes specified in Section 250-50 are available. These requirements are identical to the requirements for electrodes for separately derived systems. Basically, if available, the effectively grounded structural metal steel of a building can be used. Also an effectively grounded metal water pipe, if available, could be used. A grounding ring or concrete encased electrode, if available, should be used. The electrode chosen should be as near as practical and preferably in the same area. Only resort to using a made electrode if any of the electrodes specified in Section 250-50 (a) through (d) are not available.

Where shielded cables or conductors are used, the shields shall be grounded. Again, it would be important here to follow the manufacturer’s instructions with regards to the types of cable assemblies specified in the control drawing and any shielded cable grounding requirements. This also is important to maintaining the intrinsic safety characteristic of the circuits in some of these systems. In hazardous locations, intrinsically safe apparatus shall be bonded in accordance with the requirements of Section 250-100. It should be noted here that in Section 250-100 this bonding method is required "regardless of the voltage." Where metal raceways are used for intrinsically safe circuits and extend through nonhazardous locations, the bonding requirements of Sections 501-16(a), 502-16(a), 503-16(a), or 505-25 are applicable.

Sealing Requirements
Conduits and cables are required to be sealed to minimize the passage of gases, vapors, or dusts to areas that are not classified. The seals shall not be required to be explosionproof or flameproof. Seals are not required for enclosures that contain only intrinsically safe apparatus, except as required by Section 501-5(f)(3) for drainage.

Intrinsically safe wiring that enters a panel assembly is usually required to enter at a specific location in that assembly. There usually is a separate wiring compartment for the intrinsically safe circuits or wiring and physical barriers for maintaining the required separation. Terminal and junction locations are required to be identified by Section 504-80(a). (See Figures 5 and 6).

Labeling Requirements
When installing intrinsically safe wiring, circuits, and systems, there are requirements for labeling the raceways, cable trays, enclosures and open wiring. This labeling shall be suitable for the environment in which the labels are installed. Consideration should be given to the effects of chemicals and weather. The raceways, cable trays, and open wiring for intrinsically safe systems shall be identified with permanently affixed labels with the wording "Intrinsically Safe Wiring" or the equivalent. This labeling shall be visible after installation and placed so the wiring is readily traced through the entire length of the installation. The Code requires that spacing between these labels not exceed 25 feet (7.62 m). In underground installations, conduits or raceways are not required to be labeled, but at points of emergence from the earth the labeling is required to continue. See Section 504-80(b). Without these labels, inspection agencies cannot determine that the installation is in compliance with the Code. (See Figures 7 and 8)

Installation and inspections of intrinsically safe circuits, systems, and wiring can be challenging to even the seasoned veteran. Always consult your local authority having jurisdiction if in doubt as to the requirements of the National Electrical Code regarding the installation and use of intrinsically safe wiring in hazardous (classified) locations. If in doubt about the requirements of the manufacturer’s installation instructions or control drawing(s), call the manufacturer. Effective communication is essential in this industry for continuing growth of electrical knowledge and safe electrical installations.

With the protection techniques that are located in Article 505, new technology for intrinsically safe systems is available and should be further considered when selecting this protection technique.

Education Department Manager for the IAEI. Johnston was formerly electrical field inspections supervisor for the city of Phoenix, Arizona. He is fully certified in many areas. He is a member of IBEW. He achieved both journeyman E-2 and master electrician E-1 licenses in the state of Connecticut. Additionally, he holds all IAEI certifications. He also holds ICBO Electrical Inspections Certifications.

Home  | Join | Advertising | Seminars | Contact Us | Privacy Statement | Legal Notices
Copyright © 1997-2008 IAEI. All Rights Reserved.